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3.13 Understanding The Different Clock Speeds
The following chart shows the different CPU, mainboard and PCI fre-
quencies. As you may have noticed, all the Intel Pentium CPUs use an
internal clock multiplier(x3, x2.5, x2 or x1.5 Motherboard speed).
Pentium
Mainboard
PCI Bus
Internal Clock
Clock
Clock
75 MHz
50 MHz
25 MHz
90 MHz
60 MHz
30 MHz
100 MHz
66.67 MHz
33 MHz
120 MHz
60 MHz
30 MHz
133 MHz
66.67MHz
33 MHz
150 MHz
60 MHz
30 MHz
166 MHz
66.67 MHz
33 MHz
In the table above, the 50, 60 and 66.67 MHz figures are oscillator speeds
that establish the external clock speed. The PCI Bus clock speed is fixed at
one half of the mainboard clock speed. The 150MHz Pentium when set for
2.5x mode will achieve a 30MHz PCI bus speed but if it is set for 3x mode
you will get a slower 25MHz PCI bus speed.
